How Low can It goes?

当一回书呆子，用 SP500 earning forecase 估计市场能够走到多低？多少PE是合适的呢？17？ 20？


S&P Trailing PE Ratio
Quarter Earnings 10 12 15 17 20 25 30
12/31/2011 $72.20 722 866 1,083 1,227 1,444 1,805 2,166
09/30/2011 $70.40 704 845 1,056 1,197 1,408 1,760 2,112
06/30/2011 $68.06 681 817 1,021 1,157 1,361 1,702 2,042
03/30/2011 $64.95 650 779 974 1,104 1,299 1,624 1,949
12/31/2010 $62.09 621 745 931 1,056 1,242 1,552 1,863
09/30/2010 $61.69 617 740 925 1,049 1,234 1,542 1,851
06/30/2010 $60.93 609 731 914 1,036 1,219 1,523 1,828
03/30/2010 $59.44 594 713 892 1,010 1,189 1,486 1,783